Aboard The T.S.G.B. Day: 34
Determining the wind’s true direction is always a bit of a challenge at first. The wind that is felt across
the deck is the apparent or relative wind and not the true wind, since the ship is also moving. For
example, if there were no wind at all, we would still feel a breeze on our faces since the ship is moving
forward. If the ship is moving at 15 knots, then the relative wind we would feel would be 15 knots
from dead ahead. Calculating the wind speed is easy if there is no wind, or if we were sitting still.
When we are moving, and the wind is blowing, we have to calculate the true wind by adding and subtracting vectors. A vector is just a fancy
word for a line on a paper that represents direction and speed. By adding the relative wind to the ship’s heading (red) and course vector (green), we can then determine the true direction
and speed of the wind (blue). Below is a picture of a wind plot, so you can see the vectors and how we use them in calculating the wind.
In addition to calculating true wind, we can determine true wind by using the ‘seaman’s eye’, and in fact, we encourage our cadets to use this method to check their vector
calculations. This describes a process in which we simply look at the seas and waves, and estimate the direction from which they are coming and the height of the waves. Since ocean
waves are generated by wind, there’s a close relationship between wind and wave direction. Just observing the sea state is a good indication of the speed (force) of the wind.
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